IB Chemistry Topic 2: Atomic structure
2.1 The atom
- Protons and neutrons are present in the nucleus of an atom; electrons are in orbits or energy levels around the nucleus.
| · The relative masses and relative charges of the sub-atomic particles are: | Relative mass | Relative charge | |
| Proton | 1 | + 1 | |
| Neutron | 1 | 0 | |
| Electron | 5 ´ 10–4 | –1 |
- Atomic number (Z) = number of protons. It is the fundamental characteristic of an element.
- Mass number (A) = number of (protons + neutrons).
- Isotopes are atoms with the same atomic number, different mass numbers OR the same number of protons, but different numbers of neutrons.
- For a species :
number of protons = Z
number of electrons = Z – q
number of neutrons = A – Z.
- Isotopes differ in physical properties that depend on mass such as density, rate of diffusion, etc. This difference is very significant for the isotopes of hydrogen as deuterium has twice the mass of the more abundant. As isotopes have the same electron arrangement they have the same chemical properties.
- Examples of the uses of radioisotopes: C-14 in radiocarbon dating, Co-60 in radiotherapy and I-131, and I-125 as medical tracers.
2.2 The mass spectrometer
- Stages of operation: vaporization of the sample, ionization to produce M + ions by electron bombardment, acceleration of positive ions by an electric field, deflection of ions by magnetic field perpendicular to their path, detection of ions.
- Degree of deflection depends on the charge/mass ratio: the smaller the mass or the greater the charge: the greater the deflection.
- For an element, the mass spectrum gives two important pieces of information: the number of isotopes, and the abundance of each isotope. This allows the relative average atomic mass, Ar to be calculated.
- Relative atomic mass (Ar) of an element is the average mass of an atom according to the relative abundances of its isotopes, on a scale where the mass of one atom of is 12 exactly.
- For example for Cl which has two isotopes (75 %) and (25 %).
- For a molecule, the peak with the largest mass represents the molecular (parent) ion and its mass gives the relative molecular mass (Mr) of the compound. The fragmentation pattern can help determine the molecular structure (see Chapter 12).
2.3 Electron arrangement
- The electromagnetic spectrum includes waves in order of decreasing frequency/energy, γ rays,
X-rays, ultraviolet radiation, visible light, IR radiation, microwaves, and radio waves. (See Table 3 of the IB Data booklet). - Frequency (f) and wavelength (λ) are related by: c (speed of light) = fλ.
- The energy of a photon (Ephoton) is related to the frequency (f) of the radiation by Planck’s equation:
Ephoton = hf (The equation is given in Table 1of the IB Data booklet).
h is Planck’s constant (Table 2 of the IB Data booklet).
- A continuous spectrum contains the light of all wavelengths in the visible range.
- A line spectrum consists of a few lines of different wavelengths/frequencies.
The lines in an emission spectrum are produced by excited electrons falling from higher to lower energy levels: ΔEatom = hf = hc/λ.
As the energy levels converge at higher energy as they are further from the nucleus; the lines in the spectrum also converge at higher energy/frequency.
· The hydrogen spectrum: Series Region Electron falls to Lyman UV n = 1 Balmer Visible n = 2 Paschen IR n = 3– The ionization energy of hydrogen corresponds to the convergence limit of the Lyman series.
- The electron arrangement indicates the number of electrons in each energy level.
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Element Electron arrangement Element Electron arrangement H 1 Na 2, 8, 1 He 2 Ar 2, 8, 8 Li 2, 1 K 2, 8, 8, 1 Ne 2, 8 Ca 2, 8, 8. 2 12.1 Electron configuration
- The electron configuration of an atom describes the number of electrons in each energy sub-level.
- Evidence for the existence of main energy levels and sub-levels comes from graphs of first ionization energies of successive elements or successive ionization energies of the same element.
- The first ionization energy is the minimum energy required to remove one mole of electrons from a mole of gaseous atoms to form a mole of univalent cations in the gaseous state. It is the enthalpy change for the reaction: X (g) ¯ X + (g) + e–.
- Large increases in successive ionisation energies of an atom occur when an electron is removed from a different energy level. Smaller increases occur when an electron is removed from a different sub-level. A very small jump occurs when there is a change from a p4 to p3 configuration as paired electrons are easier to remove than unpaired electrons.
- The main energy levels (in order of increasing energy) are identified by integers,
n = 1, 2, 3, 4… Each main energy level contains n sub-levels and n2 - The sub-levels (in order of increasing energy) and orbitals are identified by letters: s, p, d, f,
· Orbitals are regions in space in which an electron may be found in an atom. They have characteristic shapes. s orbitals are spherical and p orbitals are dumb-bell shaped. There are corresponding p orbitals orientated along the x and z axis. s orbital py orbital
- Pauli Exclusion Principle states that only electrons with opposite spin can occupy the same orbital.
- The Electrons-in-boxes notation is used to describe the number of electrons in each orbital. Each orbital is represented by a box and each electron by an arrow which represents the direction of its spin.
The relative energies of the sub-levels and their composition are summarised.
Level Sub-level Maximum no. of electrons in sub-level Maximum no. of electrons in level n = 4 4f 14 (seven f orbitals) 32 4d 10 (five d orbitals) 4p 6 (three p orbitals) 4f 14 (seven f orbitals) n = 3 3d 10 (five d orbitals) 18 3p 6 (three p orbitals) 3s 2 (one s orbital) n = 2 2p 6 (three p orbitals) 8 2s 2 (one s orbital) n = 1 1s 2 (one s orbital) 2 A useful mnemonic to the order of filling orbitals. Follow the arrows to see the order in which the sub-levels are filled.
1s, 2s, 2p, 3s, 3p, 4s, 3d. 4p, 5s, 4f, 5d, 6p, 7s…
- The Aufbau principle states that orbitals with lower energy are filled before those with higher energy.
- Hund’s rule states that every orbital in a sub-level is singly occupied with electrons of the same spin before any one orbital is doubly occupied.
The number of electrons in a sub-level is represented by a super script number.
Element Electron configuration Element Electron configuration H 1s1 Sr [Ar] 3d14s2 Li 1s22s1 Cr [Ar] 3d54s1 B 1s22s22p1 Ni [Ar] 3d84s2 Na 1s22s22p63s1 Cu [Ar] 3d104s1 Note the exceptional configuration of copper and chromium.
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Topic 2: Atomic structure definitions
Acceleration (in a mass spectrometer) The stage where the positive ions are attracted to negatively charged plates.
Alpha decay The emission of an alpha particle from the nucleus of a radioactive atom. Alpha decay results in a decrease in the atomic number of the element by two and a decrease in the mass number by four.
Alpha particle An alpha particle is a helium nucleus emitted by a nucleus undergoing
alpha decay.Anions A negatively charged ion so called because they are attracted to the anode during electrolysis. They have more electrons than the parent atoms.
Atom The smallest particle of an element that can take part in a chemical reaction. All atoms of the same element have the same number of protons in the nucleus.
Atomic number The number of protons in the nucleus of an atom.
Atomic theory The theory that all substances are composed of atoms (that cannot be created or destroyed).
Aufbau principle This states that orbitals with lower energy are filled before those with higher energy.
Balmer series A series of lines in the emission spectrum of hydrogen in the visible region. The lines are produced when an electron falls down to the second-lowest energy level (n = 2).
Beta (β) particle A high-speed electron ejected from the nucleus. It is produced from the decay of a neutron with a proton, the other decay product, remaining in the nucleus.
Bohr theory A classical model of atomic structure with the electron orbiting the nucleus in circular energy levels or orbits.
Cancer A malignant growth or tumour caused by abnormal and uncontrolled cell division. It spreads to other parts of the body through the lymphatic system or the blood stream.
Carbon-14 dating A technique for determining the age of archaeological artefacts. It is based on the rate of β decay of the isotope found in materials of biological origin.
Cations A positively charged ion, so called because it is attracted to the cathode during electrolysis. They have less electrons than their parent atoms.
Compound A substance formed by the chemical combination of two or more chemical elements in fixed proportions.
Continuous spectrum An emission spectrum with all the wavelengths or frequencies of visible light.
Convergence This occurs when the lines in an emission spectrum become progressively closer to each other (at higher frequency or smaller wavelength) and finally merge. The convergence limit corresponds to the ionization with the electron in an energy level outside the atom.
Deflection (in a mass spectrometer) The stage where positive ions are deflected by a magnetic field placed at right angles to their path. The amount of deflection is proportional to the charge/mass ratio.
Degenerate orbitals Orbitals which have the same energy.
Detection (in a mass spectrometer) The stage where the number of positive ions with a particular charge/mass ratio is measured and recorded.
Deuterium The isotope of hydrogen with a single neutron in the nucleus.
Diffraction A change in direction of a wave as it passes through an opening or around a barrier. The wave properties of the electron were demonstrated when they were diffracted by a solid crystal.
d orbitals There are five d orbitals in every energy level where n ≥3.
Effective nuclear charge The nuclear charge as experienced by a particular electron. It is smaller than the actual nuclear charge due to the shielding of the nucleus by inner electrons and inter-electron repulsion.
Electromagnetic wave A wave of oscillating electric and magnetic fields that can move through a vacuum with the speed of light.
Electromagnetic spectrum The range of electromagnetic radiation or waves including, in order of decreasing frequency, g rays, X-rays, UV radiation, visible light, IR radiation, microwaves, and radio waves. (See Table 3 of the IB Data booklet).
Electron Negatively charged particles present in all atoms and located in orbitals outside the nucleus. The relative mass of the electron is 5 ´ 10–4.
Electron affinity (first) The enthalpy change when one mole of gaseous atoms accepts a mole of gaseous electrons under standard conditions: X (g) + e– g X– (g).
Electron configuration A shorthand method for describing the number of electrons in each energy sub-level.
Electrons-in-boxes notation A diagram which describes the number of electrons in each orbital. Each orbital is represented by a box and each electron by an arrow which represents the direction of its spin.
Electron transition The movement of electrons from one orbital to another. A transition from a higher to a lower energy level produces an emission spectrum. A transition from a lower to higher energy level produces an absorption spectrum.
Element A substance that cannot be decomposed or broken down into simpler substances by chemical methods; all the atoms of an element contain the same number of protons.Emission spectroscopy The study of spectra produced as excited gaseous atoms or molecules lose energy and fall to lower energy levels.
Energy levels The allowed energies of electrons in an atom (or molecule). Electrons fill the lower energy levels, or shells, closest to the nucleus, before occupying higher levels further from the nucleus.
Excited state The state of an atom or molecule when it is raised to a higher energy level above the stable ground state.
Flame test An analytical technique in which a small sample of a metal compound is placed into a hot Bunsen flame on a clean Nichrome or platinum wire. The sample emits light at wavelengths characteristic of the metal ion.
Frequency The number of complete waves passing a point per second.
Gamma waves/radiation (or rays) High energy electromagnetic radiation of high frequency/short wavelength emitted by changes in the nuclei of atoms.
Genetic material The material, found in the nucleus, mitochondria and cytoplasm of a cell, which plays a fundamental role in determining its structure and nature.
Ground state The state with lowest energy.
Heisenberg’s uncertainty principle This states that it is impossible to know both the exact position and the exact momentum of an object at the same time.
Hund’s rule This states that every orbital in a sub-level is singly occupied with electrons, with the same spin, before any orbital is doubly occupied.
Ionization (in a mass spectrometer) The stage where gaseous atoms or molecules are hit with high-energy electrons to knock out electrons and produce positively charged ions: X (g) + e– ® X– (g) + 2e–.
Ionization energy (first) The minimum energy required to remove one mole of electrons from a mole of gaseous atoms to form a mole of univalent cations in the gaseous state. It is the enthalpy change for the reaction: X (g) ® X + (g) + e–.
Isoelectronic Species (atoms/ions/molecules) that have the same number of electrons.
Isotope Atoms of the same element (and so with the same atomic number, Z) with different numbers of neutrons (and so different mass number, A).
Isotopic abundance The proportions of the different isotopes of an element.
Line spectrum A line spectrum is an emission spectrum with only certain frequencies. It is produced by excited atoms and ions as they fall back to a lower energy level.
Lyman series A series of lines in the emission spectrum of hydrogen in the ultra-violet region. The lines are produced when an electron falls down to the ground state (n = 1).
Mass number The sum of the protons and neutrons in the nucleus of the atom or ion.
Mass spectrometer An instrument in which gaseous atoms or molecules are ionized, accelerated by an electric field and then deflected by a magnetic field. The amount of deflections depends on the charge/mass of the positive ion.
Mass spectrometry This can be used to analyse the isotopic abundances of an element or the molecular structure of a compound. It can also be used to determine the relative atomic mass of an element or the relative molecular mass of a compound.
Mass spectrum The output plot from the mass spectrometer. It is a bar graph with abundance on the vertical axis and mass/charge ratio on the horizontal axis.
Molecule A group of atoms held together by covalent bonds.
Momentum The product of the mass and velocity of an object (p = mv).
Neutron A neutral particle found in the nucleus of atoms. It has approximately the same mass as the proton.
Noble gas configuration A configuration with an octet of electrons in an energy level (ns2np6), or a pair of electrons in the first shell (1s2).
Nuclear charge The nuclear charge is the total charge of all the protons in the nucleus of an atom.
Nucleon A particle in a nucleus, either a neutron or proton. The nucleon number is the same as the mass number.
Nucleus The central part of an atom containing protons and neutrons.
Nuclide A nuclide is a type of atom whose nuclei have specific numbers of protons and neutrons (nucleons).
Occam’s razor The principle attributed to the 14th-century logician and Franciscan friar William of Occam. It states that “Entities should not be multiplied unnecessarily.” This can be expressed as: “when you have two competing theories that make exactly the same predictions, the simpler one is the better.”
Octet A set of eight electrons in the outer energy level.
Octet rule Atoms (other than hydrogen and helium) typically fill their outer energy level with eight electrons (an octet) when they form compounds.
Orbit The circular path of an electron around the nucleus (in the Bohr theory).
Orbital A region in space in which an electron may be found in an atom or molecule. Each atomic orbital can hold a maximum of two electrons with opposite spins.
Paschen series A series of lines in the emission spectrum of hydrogen in the infrared region. It is produced when an electron falls down to the third-lowest energy level (n = 3).
Pauli Exclusion Principle This states that only electrons with opposite spin can occupy the same orbital.
Photon A ‘packet’ or quantum of electromagnetic radiation. The energy of the photon (Ephoton) is related to the frequency (f) of the radiation by Planck’s equation.
Planck’s Equation Ephoton = hf (The equation is given in Table 1 of the IB Data booklet). h is Planck’s constant (its value is given in Table 2 of the IB Data booklet).
p orbital A dumb bell-shaped atomic orbital. There are three p orbitals of the same energy, orientated in the x, y and z directions in a p energy sub-level.
Promotion The movement of an electron from a lower energy level to a higher energy level.
Proton Positively charged particle found in the nuclei of all atoms. It has approximately the same mass as the neutron. It has an equal and opposite charge to the electron.
Quantization The concept that energy exists in the form of small ‘packets’ called quanta.
Quantum (plural quanta) A packet of energy.
Radiation The transmission of energy by means of an electromagnetic wave; or the emission of particles from the nucleus of a decaying atom.
Radiotherapy The treatment of cancer and other diseases with ionizing radiation.
Relative atomic mass The average mass of an atom of an element, according to relative
abundances of its isotopes in a naturally occurring sample, on a scale where the mass of one atom of is 12 exactly.Shells The main energy levels of an atom where the electrons are located.
Shielding (of electrons) The reduction of the electrostatic force of attraction between an electron and the nucleus due to the presence of electrons in the energy levels closer to the nucleus.
s orbital A spherically symmetrical atomic orbital. There is one s orbital in the s sub-level. It is the lowest energy orbital in a given energy level.
Spectral line A particular wavelength/frequency of light emitted (or absorbed) by an atom, ion (or molecule).
Spectral series A group of related lines in the emission (or absorption) spectrum of a substance. The lines in a spectral series occur when all the transitions occur between one particular energy level and a set of different energy levels.
Spectroscope An instrument for examining the different wavelengths present in electromagnetic radiation.
Sub-atomic particles The particles – electrons, protons and neutrons – from which all atoms are made.
Sub-level A subdivision of an electron energy level. Sometimes referred to as a sub-shell.
Valence electrons The electrons in the outermost energy level of an atom or ion. They are generally involved in bonding.
Wavelength The distance between the peaks (or troughs) of one complete wave.
Wave-particle duality A concept from quantum theory that all matter exhibits both wave and particle properties.
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